The Control of Breathing during Weaning from Mechanical Ventilation (16)

Crs (a measure of the inspiratory elastic load) does not account for the pressure which the respiratory muscles must generate to oppose intrinsic PEEP, while VP6 reflects only the expiratory resistance of the respiratory system but not the inspiratory resistive load. Finally, none of these weaning indices would detect a cardiovascular limitation to weaning.
Aside from these methodologic concerns, there is another explanation for the apparent similarity in mechanical loads between the two patient groups. Based on the predefined weaning outcome criteria* all patients in group 2 were either tachypneic or complained of intense shortness of breath. Furthermore, their tidal volumes were significantly reduced, as is commonly seen during elastic loading, threshold loading, and after experimental respiratory muscle fatigue.- A fall in Vt reduces the elastic pressure cost of breathing and results in alveolar hypoventilation unless respiratory rate increases to compensate for changes in Ve and Vd/Vt. C02 retention in patients with weaning-induced distress implies that the respiratory rate did not increase sufficiently to maintain C02SB near C02RT.

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